What is Responsivity?
Responsivity is a measure of input-output gain of the detector in a fibre optic system. In the case of a photodetector, it is the measure of electrical output per optical input. Mostly photodetectors are linear functions of input incident power. The responsivity of a photodetector is typically expressed as units of amperes per watt or volts per watt of radiant power.
It is typically highest in a wavelength region where the photon energy is above bandgap energy, then declining sharply in the bandgap region, where absorption decreases. It can be derived as follows:-
- Rλ= Responsivity
- IP = Output Photo Current
- P = Incident Optical Power
Since hυ = Energy of Photon,
P = rp hυ
- rp = Photon flux = P/hυ = Photons/sec
Hence the electron rate comes out to be,
re = ηrp = ηP/hυ
Therefore, the output current is
Ip= eηP/hυ = eηλ/hc = eηλ/1.24
h = Plank’s Constant = 6.63 × 10-34 joule-sec
The unit of Responsivity is Amperes per watts(A W−1) or volts per watts (V W−1).
Responsivity of Typical Photodetectors
The relationship between responsivity and wavelength can be understood from the table below. The unit of wavelength is in nanometers, and the unit of responsivity is ampere per watts.
The photodetector's responsivity depends on the wavelength of incident radiation and the material's bandgap. The below-given diagram shows the responsivity vs wavelength graph. The different range of responsivity is shown: maximum, average, and minimum.
Responsivity Questions in Gate Exam
Question 1: The quantum efficiency of the photodiode is 50% with a wavelength of 0.90* 10-6. Determine the responsivity of photodiodes.
Answer: 0.36 A/W
Question 2: Determine the wavelength of the photodiode having a quantum efficiency of 40% and Responsivity of 0.304 A/W.
Answer: 0.44 μm μ
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